In some campaigns, everyone should get a chance to shine, and power level isn't as important as ability to do something interesting. In others, some characters are simply stronger, tougher, or more powerful, and "weight classes" exist.
Mack Atlas might be the strongest man in the world. He even has an aspect which says so: World's strongest superhuman. So what happens if he gets into a lifting contest with Brawl, whose powers of carbon alchemy give him a similar form of super-strength?
If both characters have nonspecific power facts like "super-strength", then it comes down to their respective rolls. These rolls will be modified by their Stunts, aspect invocations, and so forth. For example, Mack has a stunt that gives bonuses to Overcome when lifting, and can invoke "World's strongest superhuman", giving him a clear advantage.
If it's been established (through power facts or in play) that Brawl has only 10% of Mack's strength, success is very unlikely and there's point in rolling to see who can lift more. But even when one character is most powerful in a single area, that doesn't mean you should never roll. A highly agile character can defeat a super-strong character by making the most of his agility through narration, situation aspects, and stunts. A costumed vigilante can defeat a powerful paragon with the right glowing rock, because he has the right power facts that gave him access to that rock. What matters is how the conflict is presented in the fiction.